The first objective of this research project is to analyze in detail the cellular architecture of the hypothalamus of the guinea pig and of the rhesus monkey for the purpose of producing cytoarchitectonic atlases to be used as guides for experimental neuroanatomical and physiological investigations of the organization and functions of the hypothalamus. The atlas will use photomicrographs of cell-stained sections of celloidin embedded brains cut in frontal, sagittal and horizontal planes. The photomicrographs will be of sufficient magnification (about 30x) and at close enough intervals (about 90-l50 micron apart) to provide a detailed appreciation and knowledge of the complexities of hypothalamic cytoarchitecture. The second objective is to study the hypothalamus of a number of species which are most commonly used for neuroendocrinological and other physiological investigations to determine whether or not cytoarchitectonic sex differences exist, as has been reported for the medial preoptic area of the rat. This study will begin with the guinea pig and rhesus monkey and proceed to the cat, rabbit, mouse, hamster and, in confirmation of the published data, also to the rat. At least five young adult female and five young adult male animals will be used for each species. The cytoarchitectonic characteristics of all hypothalamic cell groups and areas will be compared in the two sexes. Any apparent cytoarchitectonic differences will be further investigated with planimetry and/or cell counts.